Engine emission control systems may include one or more exhaust catalysts to address the various exhaust components. These may include, for example, three-way catalysts, NOx storage catalysts, light-off catalysts, SCR catalysts, etc. Engine exhaust catalysts may require periodic regeneration to restore catalytic activity and reduce catalyst oxidation. For example, catalysts may be regenerated by injecting sufficient fuel to produce a rich environment and reduce the amount of oxygen stored at the catalyst. As such, fuel consumed during catalyst regeneration can degrade engine fuel economy. Accordingly, various catalyst regeneration strategies have been developed.
One example approach is shown by Georigk et al. in U.S. Pat. No. 6,969,492. Therein, an emission control device includes catalytic converter stages generated by at least two catalysts arranged in series. Specifically, the catalytic stages include a three-way catalyst arranged in series with (e.g., upstream of) a NOx reduction catalyst. The different ammonia storage performance of the different catalysts enables NOx reduction to be improved and reduces the need for catalyst regeneration. Another example approach is shown by Eckhoff et al. in WO 2009/080152. Therein, an engine exhaust system includes multiple NOx storage catalysts with an intermediate SCR catalyst, and an exhaust air-to-fuel ratio is continually alternated between rich and lean phases based on differences between an air-to-fuel ratio upstream of a first NOx storage catalyst and an air-to-fuel ratio downstream of a second NOx storage catalyst.
However, the inventors herein have identified potential issues with such approaches. Catalyst regeneration strategies are not only dependent on the specific configuration and nature of the different exhaust catalysts in the emission control device, but for engine systems wherein the engine can be selectively deactivated responsive to idle-stop conditions, the regeneration is also affected by the idle-stop operations performed during a vehicle drive cycle. This includes, for example, a number, frequency, and duration of the idle-stop operations performed during the vehicle's drive cycle. In particular, during an idle-stop when the engine is deactivated and fuel is shut off for the shut-down, the engine still spins a few more times. This spinning pumps air over an exhaust three-way catalyst, causing the catalyst to become oxidized and degrading its ability to reduce NOx when the engine is reactivated. Likewise, before the engine is restarted from idle-stop, the engine is spun a few times, providing another opportunity during which air can be pumped over the exhaust catalyst. While enrichment can be used to quickly regenerate the three-way catalyst upon engine reactivation, the enrichment leads to a fuel penalty. In addition, delays in engine restart can degrade engine performance.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising, during engine running, flowing exhaust gas through a first, upstream catalyst and then a second, downstream catalyst to store at least some exhaust ammonia on the first catalyst. The method further comprises selectively deactivating the engine in response to an idle-stop and during an engine restart from the idle-stop, adjusting regeneration of a third catalyst upstream of the first catalyst based on an ammonia content of the first catalyst. In addition, during high engine loads, regeneration of the second catalyst can also be adjusted based on the ammonia content of the first catalyst. In this way, ammonia generated during stoichiometric engine operation can be stored on the first exhaust catalyst and advantageously used to reduce exhaust NOx species during an engine restart from idle-stop conditions while the second and third exhaust catalysts are regenerated.
In one example, an idle-stop engine may be configured with a common exhaust manifold underbody. The underbody may include a first, SCR exhaust catalyst coupled upstream of, and in face-to-face brick contact with a second, three-way exhaust catalyst. As such, each of the first and second exhaust catalysts may be downstream of a third close-coupled three-way exhaust catalyst. During engine operation, ammonia generated by the third exhaust catalyst can be stored in the first, SCR catalyst, and retained thereon while the engine is deactivated responsive to idle-stop conditions. An air-to-fuel ratio during an engine operation prior to the idle-stop may be adjusted to be stoichiometric, or richer than stoichiometry, to store a desired amount of ammonia at the first catalyst by the time an idle-stop is performed and the engine is shut down. By storing the generated ammonia on the first, SCR catalyst, ammonia storage on the second three-way catalyst is reduced, thereby also lowering unwanted oxidation of ammonia to NOx at the second catalyst during the idle-stop. During a subsequent engine restart, the ammonia retained on the first, SCR catalyst may be used to reduce NOx species, while an air-to-fuel ratio is adjusted based on the ammonia content remaining on the first, SCR catalyst.
The ammonia content may have changed during the idle-stop. In particular, the ammonia content may have changed based on a duration of the idle-stop as well a degree of catalyst cooling or heating incurred during the idle-stop. As such, cooling of the first SCR catalyst may increase the catalyst's ammonia storage capacity until a threshold temperature is reached, allowing it to store more ammonia. However, as the temperature of the first catalyst cools below the threshold temperature, the ammonia storage capacity of the catalyst may start to fall. Thus, if ammonia was stored on the first catalyst, as the temperature of the first catalyst falls below the threshold temperature during the idle-stop, some of the stored ammonia may be released, changing the ammonia content of the first catalyst by the time an engine restart from idle-stop is requested. Additionally, when exhaust is flowing through the emission control device, it carries heat away from the catalysts, allowing the ammonia storage capacity of the SCR catalyst to be increased. Then, when the engine is stopped during the idle-stop, the SCR catalyst temperature may temporarily increase, causing the SCR catalyst to oxidize some of the stored ammonia to nitrogen or NO using the oxygen pumped in the 2-3 engine revolutions after fuel shut-off. However, if the idle-stop is for a longer duration, the catalyst may substantially cool below the threshold temperature, causing some of the stored ammonia to be lost. In the same way, over a given vehicle drive cycle (e.g., between a time at which the vehicle operator keys on the vehicle to a time at which the operator keys off the vehicle), the engine may be idle-stopped multiple times, and the air-to-fuel ratio at an engine restart may be adjusted based on how often the engine is idle-stopped.
In this way, an air-to-fuel ratio may be adjusted while an engine is operating to charge an underbody exhaust SCR catalyst with ammonia and protect an underbody three-way catalyst from being charged with the ammonia. By using the stored ammonia during a subsequent engine restart from idle-stop, an amount of fuel required to regenerate the close-coupled and underbody three-way catalysts can be reduced, providing fuel economy benefits.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.